Generally, extension of the heat range (thermally wide-ranging) and durability of a center discharge electrode are essentially required as characteristics for the spark plug. Since the spark plug is exposed to combustion or explosion gas having a temperature of up to 2,000 degrees centigrade in high speed running of an engine, it is essential to release or disperse the heat from the spark plug, particularly from the discharge end thereof. On the other hand, carbon or composite mass deposit and accumulate on the spark plug at the periphery of the discharge electrode during idling or low speed running, which must be burned out to keep the spark plug clean. It has been acknowledged in the prior art that it is necessary to maintain the discharge end of the insulator body in the spark plug approximately within a thermal range of 450.degree.-900.degree. C. to avoid overheating and deposition of carbon or sooting. However, the temperature of the discharge end widely varies depending upon the kind of engine, running condition, fuel used, change of seasons, i.e., hot or cold surroundings and the like.
Therefore, it is essential to effectively release the heat of the spark plug discharge end transmitted from the engine for avoiding overheating or for avoiding the soot deposition in order to maintain good spark plug function under these different conditions. That is, it is necessary to maintain the discharge end within the prescribed temperature range. The discharge end temperature should not exceed the maximum allowable limit, which consequently requires the discharge end to function to eliminate overheating so that pre-ignition may be restrained in high speed running, as well as the center discharge electrode heat properties.
However, there is much to be desired in the prior art as these two different properties cannot realized in a spark plug of the prior art, i.e., it has been difficult for a spark plug provided with an appropriate temperature range (heat value) under a specific running condition simultaneously to have such properties as non-soot-deposition (selfcleaning ability), eliminating overheating and heat-resistance.
Generally in a conventional spark plug, a center electrode rod (center rod) plays a dominant role in releasing the heat of the discharge end. Thus the conventional center rod composed of a single rod of nickel alloy has come to be replaced with a copper-cored nickel alloy rod which has a copper core rod axially extending throughout the nickel rod. Those nickel alloy center rods show either an almost constant, even thermal conductance or a descending conductance despite rising temperature. Similarly, in the case of copper-cored nickel alloy center rods, no increase in thermal conductance is observed as the temperature rises. Accordingly, such conventional spark plug structures of the prior art are almost incapable of changing thermal conductivity, i.e., thermal conductance corresponding to temperature or heat conditions of the discharge end.
It is much desired by spark plug users to improve the adaptability of spark plugs to temperature changes at their discharge ends, i.e., to provide a wide thermal range.
As shown in FIG. 11, a conventional spark plug comprises a rod-like metal center electrode 29 made of a copper-cored nickel alloy rod running through the axial center bore of an insulator body at the discharge end, the stepped shoulder formed on an inner wall of the center bore receiving a flange-portion with an enlarged diameter of the center electrode, whereupon a conductive sealing glass composition 27a is applied. In such a structure, the center electrode rod 29 must be in tight contact with the insulator body 21 at high temperatures for enabling heat release from the discharge end. This requires an even, constant clearance t between the center electrode rod 29 and the insulator body 21 during manufacture (at a low temperature). With a too broad clearance t the heat will accumulate at the discharge end resulting in overheat whereas a too small clearance t will cause the insulator to break due to thermal expansion of the electrode rod 29.
Thus precise and complicated process control to maintain a prescribed clearance t is necessary for manufacturing the conventional type of spark plug provided with a rod-like center electrode, i.e., controlling center bore diameter of the insulator body, inserting and setting of the center electrode rod with an even clearance in the center bore or the like, which also hinders product cost reduction.
Now with respect to durability, the discharge center electrode sustains wear due to oxidation, lead attack through spark discharges which cannot be avoided without replacing the electrode with, e.g., a noble metal electrode. Employment of such a noble metal is, however, disadvantageous due to its high cost. The electrode sustains wear also through direct exposure to exploding gas at a high temperature and high speed.
Accordingly it is an object of the present invention to provide a novel spark plug eliminating drawbacks of the prior art as aforementioned.
Another object of the present invention is to provide a spark plug of an essentially novel structure.
A further object of the present invention is to provide a spark plug with an extended heat range.
A fourth object of the present invention is to provide a spark plug having a self-cleaning discharge end.
A fifth object of the present invention is to provide a spark plug using heat conductivity-controling material which shows higher thermal conductivity with rising temperature.
A sixth object of the present invention is to provide a spark plug provided with a resistor incorporated therein and better heat releasability from the discharge end toward a terminal rod.
A still further object of the present invention is to provide a spark plug which can be manufactured at low cost.